Digital power amplifier

ABSTRACT

To provide a digital power amplifier having high power amplification efficiency.  
     The present invention relates to a digital power amplifier in which a pair of first and second switching elements inserted between a high potential power supply line and a low potential power supply line is caused to operate complementarily, and when switching the switching element which has been switched on, dead time is provided, to control power supply to a low-pass filter. In a switching section, the first switching element, a first coil, a second coil and the second switching element are connected in series, between the high potential power supply line and the low potential power supply line. The switching section comprises a first high-speed diode in which a cathode is connected to the high potential power supply line, and an anode is connected to a node between the second coil and the second switching element, and a second high-speed diode in which a cathode is connected to a node between the first switching element and the first coil, and an anode is connected to the low potential power supply line. The node between the first coil and the second coil is connected to the low-pass filter side.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims, under 35 USC 119, priority of JapaneseApplication No.2002-248733 filed Aug. 28, 2002.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a digital power amplifier, andfor example, can be applied to an audio amplifier.

[0004] 2. Description of the Related Art

[0005] Recently, digitalization of power amplifiers (amplifiers) ismaking quick progress. Particularly, in the audio amplifiers,digitalization is remarkably adopted in full scale. As equipmentequipped with the digital power amplifier (a so-called switchingamplifier), there can be mentioned DVD players, minicomponents,television sets, personal computers, portable telephones and so forth.

[0006]FIG. 7 is a block diagram showing the configuration of thevicinity of a load (speaker) in a conventional switching amplifierhaving positive and negative two power sources.

[0007] Switching elements FET1 and FET2 including, for example, FET areconnected between power supply lines +B and −B of the positive andnegative two power sources of a power supply section formed of atransformer T, diodes D1 and D2, and power supply capacitors C1 and C2.A coil L and a capacitor C constituting a low-pass filter are connectedbetween a node between the switching elements FET1 and FET2 and a nodebetween the power supply capacitors C1 and C2, and a speaker SP as aload is connected in parallel with the capacitor C constituting thelow-pass filter.

[0008] The pair of switching elements FET1 and FET2 is basically on-offcontrolled complementarily, corresponding to one-bit digital signal (PWMsignal) obtained by converting an analog audio signal. FIG. 7 shows astate in which the switching element FET1 is switched on. By the ONaction of either one of the switching elements FET1 and FET2, powersupply current flows in the forward direction or the reverse directionwith respect to the capacitor C constituting the low-pass filter. As aresult, appropriately changed voltages across the capacitor C (voltageobtained by amplifying the initial input analog audio signal) areapplied to the load SP.

[0009] Each of the both ends of the low-pass filter coil L is fed backto an analog amplifier section (not shown), which amplifies the inputanalog audio signal, via feedback resistances R1 and R2, to stabilizethe output of the low-pass filter.

[0010] When the switching element, which has been switched on, is to beswitched, through-current flows over the both switching elements, due toa difference between the transition characteristic from on to off andthe transition characteristic from off to on, of the switching elements.

[0011] In order to prevent this, there is heretofore provided dead time,during which the both switching elements are switched off, by settingthe timing for switching off the switching element, which has beenswitched on, slightly earlier than the timing for switching on theswitching element, which has been switched off.

[0012] An unnecessary high pass is removed by the low-pass filter, tocontemplate stabilization by the feedback function. In the case of theswitching amplifier, however, there is a problem in that at the time ofa high impedance load or no load, a peak occurs in the high pass in thevicinity of the cut-off frequency of the low-pass filter, as shown inthe frequency characteristic diagram in FIG. 8.

[0013] Therefore, as shown in FIG. 9, it can be considered to provide adamper including a capacitor CD and a resistance RD in parallel with thelow-pass filter capacitor C. However, a new problem occurs in that apower loss occurs in the damper.

[0014] Moreover, as described above, dead time is provided so that thepair of switching elements FET1 and FET2 does not switch on at the sametime (so that the through-current does not flow).

[0015] However, when an MOSFET is employed as the switching element, itincludes a built-in diode (body diode), and hence the reverse recoveryis very slow. Therefore, when switching is carried out at a high speed,the switching element cannot be switched off instantaneously, and evenif the switching element itself is switched off, reverse current flowsto the built-in diode due to a carrier storage effect. As a result,through-current flows from the switching element, which is switched on,to the switching element, which should have been switched off.

[0016] As a method of preventing the through-current resulting from thedelay in the operation of the built-in diode, as shown in FIG. 10, thereis used a method in which high-speed diodes D3 and D4 for blockingreverse current are provided in series with the switching elements, andhigh-speed diodes D5 and D6 for bypassing counter-electromotive voltageare provided in parallel with the switching elements. The high-speeddiodes D5 and D6 for bypassing counter-electromotive voltage form a pathin which energy accumulated, when the power supply current flows, in thelow-pass filter coil L flows as the current due to thecounter-electromotive voltage, immediately after the switching element,which has been switched on, is switched off.

[0017] However, if the high-speed diodes D3 and D4 for blocking reversecurrent are provided, there is a problem in that a voltage loss occursdue to the high-speed diodes D3 and D4, thereby decreasing theefficiency.

[0018] Therefore, a digital power amplifier having high power amplifyingefficiency has been heretofore desired.

SUMMARY OF THE INVENTION

[0019] In order to solve the above problems, the invention according tothe first aspect is a digital power amplifier comprising at least onepair of switching sections having first and second switching elementsrespectively including a unipolar transistor, which form a pair insertedin series between a high potential power supply line and a low potentialpower supply line, in which the pair of first and second switchingelements are basically caused to operate complementarily, and whenswitching the switching element which has been switched on, dead timeduring which the both switching elements are switched off is provided,to control power supply to a low-pass filter, wherein in the switchingsection, the first switching element, a first coil, a second coil andthe second switching element are connected in series in this order,between the high potential power supply line and the low potential powersupply line, the switching section comprises a first high-speed diode inwhich a cathode is connected to the high potential power supply line,and an anode is connected to a node between the second coil and thesecond switching element, and a second high-speed diode in which acathode is connected to a node between the first switching element andthe first coil, and an anode is connected to the low potential powersupply line, and the node between the first coil and the second coil isconnected to the low-pass filter side.

[0020] The invention according to the second aspect is a digital poweramplifier comprising an analog amplifier which amplifies an input analogsignal, a low-pass filter including a coil and a first capacitor, and adigital amplifier block which converts the output of the analogamplifier to a PWM signal, and controls power supply to the low-passfilter, wherein a series circuit comprising a second capacitor and aresistance is applied as a feedback circuit which feeds-back a nodevoltage between the coil and the first capacitor of the low-pass filterto the analog amplifier, and the series circuit has a damper functionfor damping a high pass peak in the frequency response characteristic ofthe low-pass filter, which occurs when a load is not connected to thelow-pass filter, or a high impedance load is connected thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a circuit diagram showing the overall configuration of adigital power amplifier according to one embodiment;

[0022]FIG. 2 is a circuit diagram showing the inner configuration of adigital amplifier block in the embodiment;

[0023]FIG. 3 is a circuit diagram showing the inner configuration of aswitching section in the embodiment;

[0024]FIG. 4 is a diagram for explaining a path for power supply currentin the state with only one switching element switched on in theembodiment;

[0025]FIG. 5 is a diagram for explaining a current path due tocounter-electromotive voltage of a low-pass filter coil, immediatelyafter switching off the switching element, which has been switched on,in the embodiment;

[0026]FIG. 6 is a diagram showing the frequency characteristic of thelow-pass filter in the embodiment;

[0027]FIG. 7 is a circuit diagram showing the configuration in thevicinity of a low-pass filter and a switching section in a conventionaldigital power amplifier;

[0028]FIG. 8 is a diagram showing the frequency characteristic of theconventional low-pass filter;

[0029]FIG. 9 is a circuit diagram showing the configuration forsuppressing a high pass peak of the conventional low-pass filter; and

[0030]FIG. 10 is a circuit diagram showing the configuration forremoving a harmful influence of the through-current due to the built-indiode in the conventional switching element.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] One embodiment of the digital power amplifier according to thepresent invention will be described in detail, with reference to thedrawings.

[0032]FIG. 1 is a circuit diagram including a partial block, showing theoverall schematic configuration of the digital power amplifier accordingto this embodiment. FIG. 2 is a circuit diagram including a partialblock, showing a digital amplifier block and the inner configuration ofa power supply section.

[0033] In FIG. 1, a non-inverted input terminal (+) and an invertedinput terminal (−) of a high-speed analog amplifier OP having theconfiguration of an operational amplifier are respectively grounded viaa resistance R10 or R11, and an analog audio signal AUD is input to thenon-inverted input terminal, which amplifies the input analog audiosignal AUD and outputs the amplified signal. The inverted input terminalof the high-speed analog amplifier OP is a feedback terminal from thelow-pass filter side, described later.

[0034] The amplified output signal from the high-speed analog amplifierOP is provided to a digital amplifier block 10, whose detailedconfiguration is shown in FIG. 2. The digital amplifier block 10 is foroperating upon reception of the power supply from the power supplysection 20, whose detailed configuration is shown in FIG. 2.

[0035] For example, the power supply section 20 is formed of atransformer T, diodes D1 and D2, and power supply capacitors C1 and C2,as in the conventional digital power amplifier. An alternating currentpower source (for example, commercial power supply) AC is applied to aprimary winding of the transformer T. A secondary winding of thetransformer T has a center tap. One end of the secondary winding isconnected to the center tap via the anode and cathode of the diode D1and the power supply capacitor C1. The center tap of the secondarywinding is also connected to the other end of the secondary winding viathe power supply capacitor C2 and the anode and the cathode of the diodeD1, so that +B (positive power source) is formed in one of the powersupply line, and −B (negative power source) is formed in the other powersupply line.

[0036] The digital amplifier block 10 has a PWM generator 11, a driversection 12 and a switching section 13.

[0037] The PWM generator 11 comprises a triangular wave generationcircuit and a comparator, for comparing the size of the output signalfrom the high-speed analog amplifier OP with that of the triangular wavesignal generated by the built-in triangular wave generation circuit, toconvert the output signal from the high-speed analog amplifier OP to aPWM signal. Here, the PWM generator 11 forms complementary PWM signals(positive-phase and negative-phase PWM signals). The positive-phase andnegative-phase PWM signals are not in a complete inverted relation,taking into consideration the dead time for preventing thethrough-current described above.

[0038] The driver section 12 is for driving the switching elements SW1and SW2 in the switching section 13, which is shown in FIG. 3 in moredetail, corresponding to the input PWM signal.

[0039] In the switching section 13, as shown in FIG. 3, the switchingelement SW1, a coil L11, a coil L12 and the switching element SW2 areconnected in this order, between the positive power supply line +B andthe negative power supply line −B. The node between the switchingelement SW1 and the coil L11 is connected to the cathode of thehigh-speed diode D12, and the anode of the high-speed diode D12 isconnected to the negative side power supply line −B. The node betweenthe coil L12 and the switching element SW2 is connected to the anode ofthe high-speed diode D11, and the cathode of the high-speed diode D11 isconnected to the positive side power supply line +B. The node betweenthe both coils L11 and L12 is connected to one end of a low-pass filtercoil L.

[0040] One end of the low-pass filter coil L is, as shown in FIG. 1,connected to an inverted input terminal of the high-speed analogamplifier OP via a feedback resistance R1 f. The other end of thelow-pass filter coil L is connected to the inverted input terminal ofthe high-speed analog amplifier OP via a series circuit (feedbackcircuit) including a capacitor Cf and a resistance R2 f.

[0041] For example, an MOSFET is employed for the switching elements SW1and SW2. In this case, the both switching elements SW1 and SW2 may bethe one having the same conductive type (for example, both areNMOS-FET), or the one having a CMOS configuration. For the high-speeddiodes D11 and D12, for example, a schottky-barrier diode is employed,which operates at a higher speed than the built-in diode (body diode) ofthe switching elements SW1 and SW2.

[0042] The point in which the load (speaker) SP is connected in parallelwith the low-pass filter capacitor C is the same as in the conventionalconfiguration.

[0043] In this embodiment, a damper in the conventional circuit as shownin FIG. 9 for suppressing the high pass peak in the vicinity of thecut-off frequency is not provided in parallel with the low-pass filtercapacitor C.

[0044] The operation and function in the characteristic configuration inthis embodiment will be described below. At first, the operation andfunction of the switching section 13 will be described.

[0045] As shown in FIG. 4, when only the switching element SW1 isswitched on, the power supply current I11 flows, and at this time,energy is accumulated not only in the low-pass filter coil L but also inthe coil L11 connected in series with the switching element SW1.

[0046] When the switching element SWl is switched off (the off-state ofthe switching element SW2 continues) by the dead time control at thetime of switching the switching element which has been switched on, theaccumulated energy flows as current I12 as shown in FIG. 5, due to thecounter-electromotive force of the low-pass filter coil L and the coilL11. That is to say, the coil L11 attempts to flow the current due tothe counter-electromotive force thereof, and draws the current due tothe counter-electromotive force by the low-pass filter coil L towardsthe coil L11 itself. As a result, it is prevented that the current dueto the counter-electromotive force by the low-pass filter coil L flowstowards the switching element SW2 side (toward the built-in diode sideof the switching element SW2).

[0047] In other words, the coil L11 exerts the reverse currentpreventing function similar to that of the diode D4 for preventingreverse current in the conventional circuit shown in FIG. 10.

[0048] Moreover, in the case of the reverse current blocking diode D4, avoltage loss occurs. However, in the case of the coil L11, such a lossdoes not occur. For the coils L11 and L12, for example, a coil havingseveral μH is employed.

[0049] The function and operation when the series circuit including thecapacitor Cf and the resistance R2 f is applied as the feedback circuitfrom the node between the coil L and the capacitor C constituting thelow-pass filter to the analog amplifier OP will be described below.

[0050] The series circuit comprising the capacitor Cf and the resistanceR2 f also works as the damper (CD, RD) shown in FIG. 9 in theconventional circuit. By applying the series circuit comprising thecapacitor Cf and the resistance R2 f as the feedback circuit, dampingeffect is reinforced, doubled by the loop gain. As a result, a highresistance can be used as a damping resistance, thereby enablingconsiderable reduction of a power loss, as compared with theconventional circuit (FIG. 9). For example, for the resistance R2 f, onehaving several tens kΩ is employed, and for the capacitor Cf, one having100 pF is employed.

[0051] Moreover, by employing the series circuit including the capacitorCf and the resistance R2 f as the feedback circuit, the phase in thehigh pass can be suppressed to 90 degrees, and the phase which delays180 degrees at maximum in the low-pass filter can be suppressed to 90degrees, thereby enabling suppression of oscillation.

[0052]FIG. 6 shows the frequency characteristic of the low-pass filteraccording to this embodiment, and it is seen that a peak in the highpass in the vicinity of the cut-off frequency as shown in FIG. 8 issuppressed.

[0053] As described above, according to the digital power amplifier inthis embodiment, since the switching element has a built-in body diode,a voltage loss, which has heretofore occurred, can be suppressed.

[0054] Further, according to the digital power amplifier in thisembodiment, since the feedback circuit has a damper function, the damperhaving heretofore provided in parallel with the capacitor of thelow-pass filter is not required. As a result, a power loss can besuppressed as compared with a conventional circuit.

[0055] (B) Other Embodiment

[0056] In the above embodiment, explanation is given for a case wherethe switching element is an MOSFET. However, the present invention isalso applicable to cases where other unipolar transistors (for example,MESFET or MISFET) having a built-in diode are used.

[0057] In the above embodiment, a pair of switching elements is used.However, the present invention is also applicable to a digital poweramplifier including two pairs (or more) of switching elements, wherein atrouble due to the through-current by the body diode is prevented by areverse current blocking diode connected in series with the switchingelement and a high-speed diode provided in parallel therewith.

[0058] In the above embodiment, explanation is given for a case wherethe present invention is applied to a digital power amplifier fordriving a speaker, but the present invention is not limited thereto, andmay be applied to an optional digital power amplifier. The effect ofapplying the switching section according to the present invention islarge, when the load (corresponding to the entire low-pass filter andload (speaker) in the above embodiment) is an inductive load.

[0059] As described above, according to the present invention, since apoint in which a loss occurs in a conventional digital power amplifieris improved, a digital power amplifier capable of performingamplification efficiently can be realized.

What is claimed is:
 1. A digital power amplifier comprising: at leastone pair of switching sections having first and second switchingelements respectively including a unipolar transistor, which form a pairinserted in series between a high potential power supply line and a lowpotential power supply line, in which the pair of said first and secondswitching elements are basically made to operate complementarily, andwhen switching the switching element which has been switched on, deadtime during which the both switching elements are switched off isprovided, to control power supply to a low-pass filter; wherein in saidswitching section, said first switching element, a first coil, a secondcoil and said second switching element are connected in series in thisorder, between said high potential power supply line and the lowpotential power supply line, and the switching section comprises a firsthigh-speed diode in which a cathode is connected to said high potentialpower supply line, and an anode is connected to a node between saidsecond coil and said second switching element, and a second high-speeddiode in which a cathode is connected to a node between said firstswitching element and said first coil, and an anode is connected to saidlow potential power supply line, and the node between said first coiland said second coil is connected to said low-pass filter side.
 2. Adigital power amplifier comprising: an analog amplifier which amplifiesan input analog signal; a low-pass filter including a coil and a firstcapacitor; and a digital amplifier block which converts the output ofsaid analog amplifier to a PWM signal, and controls power supply to saidlow-pass filter; wherein a series circuit comprising a second capacitorand a resistance is applied as a feedback circuit which feeds-back anode voltage between the coil and the first capacitor of said low-passfilter to said analog amplifier, and the series circuit has a damperfunction for damping a high pass peak in the frequency responsecharacteristic of said low-pass filter, which occurs when a load is notconnected to said low-pass filter, or a high impedance load is connectedthereto.